The invention relates generally to cardiac pacers, and more particularly to means for preventing crosstalk between bipolar pacer leads.
There are two major pumping chambers in the heart, the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles. The atria contract in a separate action which precedes the major ventricular contraction by an interval of about 100 milliseconds (md), known as the AV delay. In the healthy heart, atrial contractions begin every 400-1,000 ms at a steady metabolically determined frequency known as the "sinus" rate, which increases automatically with exercise, the AV delay being foreshortened at higher rates.
Electrical signals corresponding to the contractions appear in the electrocardiogram. A signal known as the P-wave accompanies the atrial contraction while a signal known as the QRS complex, with a predominant R-wave, accompanies the ventricular contraction. The P and R-waves can be reliably detected as timing signals by electrical leads in contact with the respective heart chambers.
The typical implanted cardiac pacer operates by producing electrical stimulation pulses to supply missing excitation via an insulated wire (or "pacing lead") terminating in an electrode attached to the right ventricle. The R-wave can be sensed by the same lead to inhibit or trigger stimulation or to restart a timing interval as in "demand" pacing. An additional lead contacts the atrium to sense P-waves, if desired. Pacers whose ventricular stimulation is timed from the sensing of a P-wave are referred to as synchronous or "physiological" pacers since they preserve the natural sinus rate as well as the normal sequence of contractins. In AV sequential pacers, sometimes the atrial lead is also used for atrial stimulation.
There are two basic types of electrode systems used in pacing leads. Unipolar leads terminate distally in a single electrode (cathode) and employ the case of the pulse generator itself, or a conductive plate on the case, as the return electrode or ground (anode). Bipolar pacing leads, on the other hand, terminate distally in two spaced insulated electrodes connected to the pulse generator through respective wires in the pacing lead. Thus, each bipolar lead carries a positive and negative electrode for the respective chamber, and the case is not designed to form a part of the electrical circuit in this configuration.
In an AV sequential bipolar lead pacing system, bipolar pacing leads extend into the right atrium and right ventricle. In a pacer having a common ground connection, the two positive electrodes on the respective bipolar leads are tied together electrically. This shared ground connection can present crosstalk problems in both sensing and stimulation when each bipolar lead is in a different heart chamber. This is an extremely important problem to solve for physiological pacers which provide bipolar stimulation and sensing for both heart chambers with the same implanted pacer powered by a single battery.
One of the ways previously used to accomplish some measure of isolation between bipolar leads is to employ a transformer in the output stage of the pacing circuit to isolate the lead electrodes. This approach, however, has only been practical when sensing is done only one one channel. In addition, it has the serious drawback of adding a relatively bulky ineffecient component to the otherwise miniaturized pacer electronics.
In U.S. application Ser. No. 375,040, filed May 5, 1982, and assigned to the assignee of the present invention, in order to prevent crosstalk between atrial and ventricular bipolar cardiac pacer leads, separate input/output circuits for the two channels are powered by respective isolated capacitors. In one embodiment, the capacitors are resistively coupled to the pacer battery for continuous charging. In another embodiment, one side of each capacitor is normally disconnected from the pacer battery. Charging switches momentarily connect one or the other capacitor directly to the battery in response to the output of the corresponding sense amplifier.
In U.S. application Ser. No. 375,198, filed May 5, 1982, and assigned to the assignee of the present invention, interchannel crosstalk in an existing dual channel pacer designed for bipolar leads is reduced by inserting a switching circuit between the pacing leads and the pacing terminals. In one embodiment, in each channel an isolation resistance and buffer amplifier in series with the lead electrodes, respectively, are shunting during stimulation. In another embodiment, the lead electrodes are connected to a pair of differential amplifiers which are bipassed during stimulation on a given channel.
I have discovered a system that improves upon and simplifies the aforementioned isolation circuits, may be used with most existing pacers, and is useful to obviate any cross-stimulation between the two pacer channels in any correct or incorrect lead connection.